Composite membrane with integral rim

ABSTRACT

Composite membranes that are adapted for separation, purification, filtration, analysis, reaction and sensing. The composite membranes can include a porous support structure having elongate pore channels extending through the support structure. The composite membrane also includes an active layer comprising an active layer material, where the active layer material is completely disposed within the pore channels between the surfaces of the support structure. The active layer is intimately integrated within the support structure, thus enabling great robustness, reliability, resistance to mechanical stress and thermal cycling, and high selectivity. Methods for the fabrication of composite membranes are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority as a continuation application of U.S.patent application Ser. No. 11/745,449 filed May 7, 2007, which claimspriority to U.S. Provisional Patent Application No. 60/767,513 filed May7, 2006. Each of these applications is incorporated herein by referencein its entirety.

STATEMENT REGARDING FEDERALLY-FUNDED RESEARCH

This invention was funded by the National Science Foundation under GrantNo. DMI-0420147 (Phase I) and Grant No. OII-0548757 (Phase II), and bythe Department of Energy under Grant No. DE-FG02-04ER84086, bothadministered by the Small Business Innovation Research (SBIR) program.The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to composite membranes, methods for makingcomposite membranes and applications of the composite membranes. Thecomposite membranes include a porous support structure and one or moreactive layers disposed within the pores of the support structure.

2. Description of Related Art

Efficient and cost-effective membranes are needed in many applications,including separation and purification of gases, such as the purificationof hydrogen (H₂) for use in fuel cells and in point-of-use applications.As an example, it is often necessary to remove contaminant gases such ascarbon monoxide (CO) from a gas stream containing H₂. Some membranesinclude a membrane support and an active layer, where the active layeris permeable to only species that are desired to go through themembrane, commonly referred to as supported membranes. In other cases,the entire membrane body serves as a separating layer, commonly referredto as bulk membranes.

For H₂ separation, membrane active layers of metals and metal alloys,particularly those including palladium (Pd), are impervious to all gasspecies except H₂ and thereby separate the H₂ from the other gases. Suchmembranes can be fabricated in the form of self-supporting bulk foils.Although Pd-based bulk foils exhibit near-infinite selectivity for H₂,they are expensive and have poor flux due to the required foilthickness.

Active membrane layers can also be supported by porous substrates andthin Pd-based supported films can be used to increase membrane flux.However, the fabrication of thin-film Pd supported membranes that havethe required defect-free structure requires a Pd thickness of at least10 μm to 50 μm, which is too thick for many applications, such as H₂separation in portable fuel cell reformers. Furthermore, the reliabilityof supported membranes is limited by the poor mechanical integrity ofthe thin metal layers deposited onto the porous support. Further, thepoor mechanical integrity is often exacerbated by temperature cyclingand/or mechanical loads that are encountered in use. Also, the reliablesealing of thin supported membranes is also challenging and the cost ofthe manufacturing and integration of such membranes has hindered theirwidespread application.

Recently, MEMS technology has been applied to supported membranes togenerate defect-free high permeability membranes, as is reported byKarnik et al. (“Towards a palladium micro-membrane for the water gasshift reaction: microfabrication approach and hydrogen purificationresults”, Journal of Microelectromechanical Systems, February 2003, Vol.12, Issue 1, pgs. 93-100). Submicron-thick Pd “windows” produced onetched silicon wafers demonstrated large hydrogen flux as a function ofPd area and demonstrated high selectivity. However, the total area ofthe supported Pd membrane was small, limiting the total flux.Additionally, the Pd windows ruptured when subjected to transmembranepressures of about 0.5 bar, and the thermal reliability of the thin Pdfilm on Si was a problem due to the mismatch of temperature expansioncoefficients.

Although thin-film supported membranes, such those described above forH₂ separation, have been fabricated, their commercial utility has notbeen realized. Such membranes have problems related to poor adhesion ofthe Pd layer to the support, damage to the Pd layer caused by thermalcycling and susceptibility to damage from mechanical abrasion

Films of Anodic aluminum oxide (AAO) includes elongate mesopores thatextend through the entire thickness (Furneau et al, Nature, 71, p. 337(1992)), and has been utilized as a substrate for different types ofmembranes. For example, Pd films as thin as 200 nm have been sputteredonto the surface of AAO for a H₂ separation membrane, as is reported byKonno et al. (“A Composite Palladium and Porous Aluminum Oxide Membranefor Hydrogen Gas Separation”, J. Membr. Sci., Vol. 37, pp. 193-197,1988) and Mardilovich et al. (“Gas Permeability of Anodized AluminaMembranes with a Palladium-Ruthenium Alloy Layer”, Russian J. Phys.Chem., Vol. 70, pp. 514-517, 1996). The resulting membranes exhibit highselectivity and permeability for H₂. However, although these membranescould provide much thinner active layer, the active layer is still onthe membrane surface and is prone to hydrogen embrittlement andmechanical damage.

Itoh et al. (“Deposition of Palladium Inside Straight Mesopores ofAnodic Alumina Tube and its Hydrogen Permeability”, Micropor. andMesopor. Mat. and Chem. Res., Vol. 39, pp. 103-111, 2000) report that Pdwas deposited inside the pores of AAO for the fabrication of membranesfor the separation of H₂. Fabrication of these membranes involvedsputtering of a conductive contact from Pd, Pt or Ag onto one of thesurfaces of the blank AAO membrane, followed by the electrodeposition ofPd, resulting in an active layer comprised of the Pd deposited onto thecontact film on the membrane surface as well as inside the AAO pores.The method does not allow the formation of the active layer disposedentirely within the nanoporous support structure.

SUMMARY OF THE INVENTION

In view of the foregoing, it is a primary objective of the presentinvention to provide a composite membrane, where the composite membranehas improved resistance to thermal cycling. A further objective is toprovide a composite membrane having improved mechanical reliability. Itis a further objective to provide a composite membrane having improvedadhesion of the active layer to the support structure. It is anotherobjective of the present invention to provide a composite membranehaving a high permselectivity for a gas species of interest, such as H₂.It is another objective of the present invention to provide a compositemembrane that is attached to a metal rim for low cost and convenientsealing and integration of membranes into membrane modules andseparating systems.

One or more of the foregoing objectives and advantages may be realizedaccording to the present invention, which in one aspect provides acomposite membrane comprising a porous support structure and one or moreactive layers disposed within the pores of the support structure.According to this aspect, the thin active layer can facilitate highpermeance of a gas species even as the support structure can be of muchgreater thickness to provide required mechanical integrity over a widepressure range.

A composite membrane according to the present invention can include aporous support structure having a first major surface and a mutuallyopposed second major surface, the porous support structure havingsubstantially parallel elongate pore channels extending through thesupport structure from the first major surface to the second majorsurface. The membrane includes an active layer comprising an activelayer material, the active layer material being completely disposedwithin the elongate pore channels between the first major surface andthe second major surface.

According to one aspect, the porous support structure comprises anodicaluminum oxide. According to a further aspect, the active layer isspaced inwardly from each of the first and second major surfaces, suchas by at least about 1 nm.

According to one aspect, the composite membrane is a symmetric poremembrane, wherein the pore channels have a substantially constantdiameter throughout their length. The pore channels can have an averagepore diameter, for example, of at least about 1 nm and not greater thanabout 1000 nm. The porous support structure can have an averagethickness of, for example, at least about 0.1 μm and not greater thanabout 500 μm. The active layer can have an average thickness of, forexample, not greater than about 5 μm and at least about 1 nm.

According to one aspect, the composite membrane is an asymmetric poremembrane, wherein the pore channels comprise portions of varying totalporosity and/or average pore size. For example, the pore channels caninclude at least a first portion having a first average pore diameterand a second portion having a second average pore diameter, wherein thefirst average pore diameter is at least about 1 nm and not greater thanabout 1000 nm and the second average pore diameter is smaller than thefirst average pore diameter. For example, the second average porediameter can be not greater than about 100 nm, such as not greater thanabout 50 nm. Further, the second average pore diameter can be at leastabout 0.1 nm. According to one aspect, the active layer has a thicknessof at least about 1 nm and not greater than about 5 μm. The active layercan be disposed within the second portion of the pore channels, and cancomprise dense nanoplugs of the active layer material, such as Pd oralloys of Pd.

According to yet another aspect, the active layer comprises a firstactive layer material and a second active layer material that isdifferent than said first active layer material, such as where the firstactive layer material is adapted to separate a gas species and thesecond active layer material is a catalytic material. According toanother aspect, the composite membrane can include an aluminum rimdisposed around and adhered to an outer edge of the support structure,such as to facilitate sealing of the membrane within a device.

According to one aspect, the active layer or layers disposed within thesupport structure can have the same or different pore size and porosityand the same or different composition as the support structure. Theactive layer can comprise coatings of the active material in the form ofnanoplugs, nanotubes or nanoparticles disposed inside the pores of thesupport structure.

According to another aspect, the total porosity of the support structureis well-controlled to be in a range of at least about 5% and not greaterthan about 90%.

The use of anodic aluminum oxide can enable the active layer to have athickness in the range of from below about 1 nm to tens of micrometers.

For composite membranes useful for H₂ separation, the thickness of theactive layer of metal or metal alloy can be as thin as 10 nm, enablingan increase in H₂ flux by orders of magnitude as compared toconventional foil membranes or thin film supported membranes, whilemaintaining high permselectivity for H₂.

According to one aspect of the present invention, the small size of thenanostructures of active layer materials embedded within the pores alsogreatly increases their resistance to thermal cycling and/or mechanicalloads, reducing defect formation. Nanostructured materials in generalhave greater mechanical strength and greater integrity during thermalcycling as compared to microstructured composite membranes.

According to one aspect of the present invention, the location of theactive layer within the pores of the support structure can be controlledsuch that the active layer is disposed at a predetermined positionbeneath the surface of the support structure. This can be achieved, forexample, through the use of sacrificial layer(s) during manufacture ofthe composite membrane. Placement of the active layer within thethickness of the support structure, as compared to placement on themembrane surface, enables a composite membrane having increasedperformance and increased reliability. Advantages can include increasedadhesion of the active layer to the support structure, and increasedresistance to thermal cycling and mechanical abrasion damage.

According to one aspect of the present invention, the active layer iscomprised of several different materials disposed in multiple layersthat can be deposited either consecutively or concurrently. The materiallayers can be oriented either perpendicular or parallel to the pore axisto achieve desired separation performance.

According to another aspect, the composite membranes of the presentinvention can be fabricated into virtually any shape, including eitherplanar or tubular formats.

One embodiment of the present invention is directed to a method forfabricating a composite membrane. The method can include the steps ofproviding a porous support structure having a first major surface and amutually opposed second major surface, the porous support structurecomprising elongate pore channels extending through the supportstructure from the first major surface to the second major surface,providing a sacrificial layer that is at least partially disposed on thefirst major surface of the porous support structure, depositing anactive layer material within the pore channels and adjacent to thesacrificial layer, and removing the sacrificial layer from the firstmajor surface of the porous support structure. The active layer materialcan advantageously form an active layer that is completely disposedwithin the elongate pore channels between the first major surface andthe second major surface.

According to one aspect, the porous support comprises anodic aluminumoxide. Accordingly, the step of providing a porous support structure caninclude providing an aluminum metal substrate having at least one topsurface, masking a portion of the top surface to define a target area,placing the aluminum substrate in an electrolyte, and anodizing thealuminum metal to form anodic aluminum oxide by applying a voltage tothe aluminum metal. The masking step can include the use ofphotolithography. According to this aspect, the aluminum metal substratecan function as the sacrificial layer, and the step of removing thesacrificial layer can include masking at least a portion of the aluminummetal substrate to form a masked portion, wherein the masked portion isnot removed during the removal of the sacrificial layer. In one aspect,the masked portion is disposed around a peripheral edge of the poroussupport structure.

According to another aspect of this method, the step of providing asacrificial layer can include depositing a sacrificial material on atleast the first major surface, such as by electrodeposition of thesacrificial material. Further, the sacrificial material can be depositedat least partially within the pore channels, such that the formed activelayer is spaced inwardly from the first major surface. The sacrificialmaterial can comprise, for example, Cu and the active layer material cancomprise Pd, including Pd alloyed with a metal selected from the groupconsisting of Cu and Ag. The active layer material can also be depositedby electrodeposition. A second active layer material can also bedeposited within the pore channels.

According to another aspect of this method, a solvent can be added tothe electrolyte to lower the freezing point of the electrolyte, suchthat the anodization can be carried out at a reduced temperature. Forexample, the solvent can be selected from the group consisting ofalcohols, ketones and glycols. In another aspect, a material can beadded to the electrolyte to increase the anodization voltage of theelectrolyte, such that the anodization can be carried out at increasedvoltages.

One embodiment of the present invention is directed to an asymmetricporous anodic aluminum oxide structure. The structure can include afirst major surface, a mutually opposed second major surface, and aplurality of substantially parallel elongate pore channels extendingthrough the support structure from the first major surface to the secondmajor surface, the pore channels comprising at least a first portionhaving a first average pore diameter, a second portion having a secondaverage pore diameter, and a third portion disposed between the firstand second portions, the third portion having a third average porediameter, where the third average pore diameter is different than eitherof the first and second average pore diameters.

According to one aspect of this embodiment, the first and second averagepore diameters are substantially the same. According to another aspect,the first average pore diameter is greater than the second average porediameter. At least one of the first, second and third average porediameters can be at least about 1 nm and is not greater than about 1000nm. Further, an active layer can be disposed within one of the portions,such as the third portion, such as to form a composite membrane.

A further embodiment of the present invention is directed to a methodfor the fabrication of an asymmetric anodic aluminum oxide structure.The method can include the steps of providing an aluminum metalsubstrate, placing the aluminum metal substrate in an electrolyte andanodizing the aluminum metal substrate to form anodic aluminum oxide byapplying a voltage to the aluminum metal, wherein the anodizing stepincludes changing the voltage during at least a portion of applying thevoltage at an absolute rate of not greater than about 100V/s (i.e, arate that is from +100 V/s to −100V/s).

According to one aspect of this method, the rate of voltage change(dE/dt) varies during at least a portion of the anodizing step. Forexample, an initial rate of voltage change (dE/dt)₁ can be at leastabout −0.1 V/min and not greater than about −10 V/s. According to afurther aspect, a final rate of voltage change (dE/dt)₂ is differentthan the initial rate of voltage change and is at least about −10 V/minand is not greater than 0.

The foregoing method can be used to fabricate an asymmetric structurehaving a plurality of substantially parallel elongate pore channelsextending through the support structure from a first major surface to asecond major surface, the pore channels comprising at least a firstportion having a first average pore diameter, a second portion having asecond average pore diameter, and a third portion disposed between thefirst and second portions, the third portion having a third average porediameter, where the third average pore diameter is different than eitherof the first and second average pore diameters.

According to yet another embodiment, the present invention is directedto an anodic aluminum oxide structure where the structure comprises aporous anodic aluminum oxide substrate having a first major surface anda mutually opposed second major surface and pore channels disposedthrough the substrate and extending from the first major surface to thesecond major surface, an edge connecting the first and second majorsurfaces around a periphery of the substrate, and an aluminum metal rimdisposed around the peripheral edge of the substrate, the peripheraledge being directly bonded to the substrate.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic representation of the structure of acomposite membrane according to an embodiment of the present invention.

FIG. 2 is a schematic representation of composite membrane according tothe prior art (top image) and various embodiments of the presentinvention.

FIG. 3 illustrates a method for the fabrication of a symmetric compositemembrane according to an embodiment of the present invention, where anactive layer is formed after the separation of the support membrane fromAl.

FIG. 4 illustrates a method for the fabrication of an asymmetriccomposite membrane according to an embodiment of the present invention,where an active layer is formed prior to the separation of the supportmembrane from Al.

FIG. 5 illustrates a method of the fabrication of planar compositemembranes having a metal rim according to an embodiment of the presentinvention.

FIG. 6 illustrates anodization voltage reduction profiles, correspondingcurrent density for a profile according to an embodiment of the presentinvention.

FIG. 7 illustrates the thickness of a Pd layer as a function ofdeposition time for symmetric and asymmetric membranes according to anembodiment of the present invention.

FIG. 8 illustrates the flux of H₂ and Ar through a composite membrane(membrane area 0.5 cm²) according to an embodiment of the presentinvention.

FIG. 9 illustrates the temperature cycling stability of a compositemembrane according to an embodiment of the present invention.

DESCRIPTION OF THE INVENTION

The present invention is directed to a composite membrane, such as forthe selective separation of gas species. The composite membrane 100,shown in FIG. 1, includes a porous support membrane (or supportstructure) 102 and one or more active layers 110 disposed within thepore channels 108, which extend through the support structure betweenthe first major surface 104 and the second major surface 106 of thesupport structure 102. The active layer 110 can advantageously bepermselective for one or more gas species, such as H₂.

In one embodiment, the porous support structure 102 is an anodicaluminum oxide (AAO) substrate. AAO advantageously has substantiallyparallel and uniform pore channels 108 extending through the thicknessof the AAO substrate that are essentially the same diameter along thepore length, including at the location of the active layer. The porediameter of the pore channels can be at least about 1 nm, such as atleast about 5 nm, and can range up to about 1000 nm, such as not greaterthan about 300 nm. Such support structures where the pores have asubstantially uniform diameter through the thickness of the support arereferred to herein as symmetric pore membranes 100 shown in FIG. 2.

The thickness of the support structure 102 can be at least about 0.1 μm,such as at least about 10 μm, and can range up to about 500 μm, such asup to about 200 μm. In one embodiment, the composite membrane is afree-standing membrane and the support structure thickness is from about10 μm to about 500 μm. In another embodiment, the composite membrane isa membrane with Al rim 112, such as one that is attached to a peripheraledge of the support 102, as is illustrated in FIG. 1, and is furtherillustrated in FIG. 5. The thickness of the supported structure can beat least about 0.1 μm and not greater than about 200 μm.

In another embodiment of the present invention, the composite membraneis an asymmetric pore membrane 130 as shown in FIG. 2. According to thisembodiment, throughout most of the porous support thickness the supportcomprises relatively large pores and a high porosity to maximize overallflux. Accordingly, in one embodiment at least about 50% of the totalthickness and not greater than about 99.99% of the total thickness ofthe support comprise pores having a relatively large first average porediameter. The total thickness can be at least about 1 μm and not greaterthan about 500 μm, and the first pore diameter can be at least about 10nm, preferably at least about 50 nm, and not greater than about 500 nm,preferably not greater than about 300 nm. The porosity of the regionshaving the first pore diameter is preferably at least about 10%, morepreferably at least about 20%, and preferably not greater than about80%.

In the asymmetric pore membranes, the pores extending through thesupport structure have a second pore diameter that is less than thefirst pore diameter. In this regard, this layer 134 (FIG. 2) has poresdiameter preferably not greater than about 100 nm and more preferablynot greater than about 50 nm. However, for most applications the secondpore diameter should be at least about 1 nm.

The active layer according to the present invention comprises materialsthat have the desired permeation and/or separation properties defined bytheir composition, structure, morphology or all of the above, asillustrated in FIG. 2. The materials are disposed within the pores ofmembrane support structure in a variety of architectures, depending onspecific embodiments, and can take form of dense nanoscale plugs closingthe pores (e.g., nanoplugs 110), conformal nanoscale coatings on thepore walls (e.g., nanotubes), different coatings serving differentfunctions and disposed in parallel relation relative to the pore axis(nanoplug 124) or perpendicular to the pore axis (nanoplug 126). Anassembly of separate nanoparticles or other types of nanostructures canalso be utilized. These nanoscale structures disposed within the poresand between the surfaces of membrane support at a predefined locationcomprise the active layer. According to one aspect of the presentinvention, the composite membranes are different from the prior artmembranes 120 shown in FIG. 2, where active layer 122 is disposed partlyor completely on the surface of the AAO support membrane.

Preferably, the active layer 139 is at least partially disposed withinthe region of the support having the smaller second pore diameter 134 or136. This advantageously enables the active layer to have a thicknessthat is only a fraction of the support structure thickness. In oneembodiment, the active layer in an asymmetric pore membrane has athickness that is not greater than about 50 μm, preferably not greaterthan about 25 μm and even more preferably not greater than about 1 μm.For most applications, the thickness of the active layer is preferablyat least about 0.001 μm. The smaller pore diameter with the active layerdisposed therein enables reliable encapsulation of the nanoplugs,nanotubes or nanoparticles of reduced size, in turn enabling thin (aslow as about 1 μm, and preferably 0.001 μm) yet defect-free activelayer, further increasing permeability, while maintaining highpermselectivity.

According to one embodiment of the present invention, differentmaterials can be utilized for the active layer depending upon thetargeted application of the composite membrane. For example, the activelayer materials can comprise metals, including metal alloys. Preferredamong these are Pd and Pd alloys for H₂ separation. Ceramics and metaloxides such as alumina (Al₂O₃) or silica (SiO₂) can be utilized,particularly for reducing the pore size for size-selective separation.Catalytic materials, such as ZnO/Cu or others can be utilized forcatalytic separation and membrane-reactors. Polymers such as polyimidescan be utilized for olefin/paraffin separation. Salts can also beutilized in the active layer, such as solid proton electrolytes for H₂separation or oxygen-conducting solid electrolytes. Carbon nanotubes canbe utilized for size-selective separation and water filtration.

More specifically, active layer materials that are particularly usefulfor H₂ separation can include Pd and Pd alloys. Among the Pd alloys arealloys with Ag, Cu, Ru, Au, Ni, Fe, Si, Mn, Co, Sn, Pb, Y, Ce, andcombinations thereof. Metals and alloys other than Pd, such as Ta and Rucan also be utilized. Metal oxides such as SiO₂, zeolites, mixed oxidessuch as Ba—Ce—Y oxide, polymers and other materials can also be used.

According to one preferred embodiment of the present invention for H₂separation, the active layer comprises Pd or a Pd alloy. The Pd alloycan include Cu, particularly 1 mol. % to 99 mol. % Cu, and preferably 30mol. % to 50 mol. % Cu. In another embodiment, the Pd alloy can includeAg, such as 1 mol. % to 99 mol. % Ag, and preferably 20 mol. % to 40mol. % Ag.

Other materials that can be utilized for the active layer includeamorphous metal alloys (AMA), also referred to as metal glasses, whichhave been identified as an alternative to Pd for H₂ separation due totheir strength, toughness, corrosion resistance, and ability to formthin films. However, due to lower bulk permeability in comparison withpure Pd, these materials must typically be used as ultra-thin layers toachieve the required H₂ permeability, while maintaining high filmintegrity and low defect density to sustain the desired selectivity.

All amorphous metal alloys are thermodynamically metastable. When thetemperature is increased, the performance of such membranes deterioratesdue to gradual crystallization, which affects their practicalapplications. It is an advantage that the confinement of amorphous metalalloys to small pores in the composite membranes of the presentinvention can improve the thermal stability and extend their range ofapplication. For example, the AMA alloys can be selected from Zr—Ni andNi—B(P). Other amorphous metal alloys include Zr—Ni—Hf, Zr—Ni—B,Zr—Nb—Ni, Ni—Pd—P, Ni—Ru—P, Ti—Fe, Fe—B—Si, Fe—Ni—P—B, (combinations ofY, Ti, Zr and Hf with Fe, Ni, Cu, Rh and Pd), Pd—Si, Pd—Cu—Si, Zr—Pd andothers. Amorphous metals and alloys can be produced and deposited byvarious techniques, including rapid quenching of a melt, thermalevaporation, sputtering, electrodeposition, electroless deposition, ionimplantation, mechanical alloying, or by hydrogenating the crystallinealloys.

One differentiating feature of the present invention is that the activelayer materials can be deposited as nanoplugs, nanotubes, nanoparticlesor other nanostructures within a pre-determined location of the pores ofa support membrane, forming a thin yet robust and substantially defectfree active layer (FIG. 2). The active layer can have a highpermeability, high permselectivity and increased robustness that can notbe achieved with conventional supported membranes.

Deposition methods for the active layer can include, but are not limitedto, electrochemical deposition, electroless deposition, sol-geldeposition, solution impregnation, melt impregnation, polymerization,chemical vapor deposition, atomic layer deposition, plasma sputtering,thermal evaporation, vacuum deposition, and other methods know to thoseskilled in the art.

To control the depth of the active layer within the porous supportstructure, a sacrificial material layer can be used, such as shown inFIG. 3 for symmetric membranes and FIG. 4 for asymmetric membranes. Thesacrificial layer 144 can be deposited using any of the methodsdescribed above for the active layer. After deposition of thesacrificial layer, the active layer 110 in symmetric membrane (FIG. 3)or 139 asymmetric membranes can then be deposited adjacent to thesacrificial layer, followed by selective removal of the sacrificiallayer to leave nanostructures of the active layer at a preselected depthwithin the pores of the support structure. Materials that can beutilized for the sacrificial layer can include metals, salts, oxides,ceramics, polymers and other materials.

Several materials can be used as a sacrificial layer for a Pd-basedactive layer. These include metals such as Cu, Zn, Fe, Co, Ni, Ag, In,Sn, Pb, Bi and mixtures thereof, with Cu and Zn being particularlypreferred. Metal oxides can also be utilized, particularly Al₂O₃, SiO₂,TiO₂ and ZnO, with ZnO being particularly preferred. Further, polymers,and in particular conducting polymers, can also be used.

As shown in FIG. 3, one group of preferred methods for the fabricationof composite membranes for H₂ separation involves separation of the AAOmembrane 102 support from its originating Al substrate 140, followed bydeposition of conductive contacts and a sacrificial Cu 144 onto the faceof the membrane support, followed by electrodeposition of Pd to form theactive layer 110 within the pores. Another method shown in FIG. 4 canutilize the Al foil 140 as an electrical contact for electrodepositionof both the sacrificial Cu layer and the Pd active layer.

According to one embodiment shown in FIG. 5, the fabrication methodprovides the fabrication of composite membranes with an integrated Alrim 112. Using similar approach, another method provides fabrication oftubular membranes with integrated Al ends.

Support Structure and Method of its Fabrication

The present invention includes the use of a porous support structure.Preferably, the porous support comprises anodic aluminum oxide (AAO)formed by anodization (electrochemical oxidation) of aluminum (Al). Amethod for the fabrication of AAO is described, for example, in U.S.Pat. No. 6,705,152 by Routkevitch et al., which is incorporated hereinby reference in its entirety.

According to the present invention, prior to anodization to form theAAO, the Al foil is cut and rolled to reduce the thickness of the Alfoil to a thickness of not greater than about 80% of the originalthickness. After reducing the thickness, the Al foil is annealed at atemperature of at least about 200° C. and more preferably at least about350° C. The annealing is preferably a pressure-anneal, where thepressure during annealing is above atmospheric pressure, such as apressure of at least about 5,000 psi, and preferably not greater thanabout 20,000 psi, more preferably not greater than about 10,000 psi.

It has been found that this rolling and pressure-annealing treatmentadvantageously results in an Al foil having the desired thickness,surface quality and crystalline structure (from non-texturednanocrystalline structure to highly textured structure with large grainsize) in order to produce AAO support structures suitable to specificuses.

The resulting Al foil is cleaned and optionally pre-anodized to form anAAO layer, such as an AAO layer having a thickness of from about 1 μm toabout 100 μm. The AAO layer is then stripped to remove the defectivesurface layer and to provide an Al foil surface pre-patterned with poreindents.

The Al foil can then be lithographically patterned or masked on one ofboth sides, such as by using tape, varnish, compression gaskets, orsimilar means. The patterning defines the membrane size and shape. TheAl foil can then be anodized to form the support structure having thedesired thickness and pore diameter.

Anodization of an Al foil to form AAO substrates with pore diameter offrom about 20 nm to about 200 nm is a process known to those skilled inthe art. The present invention advantageously provides a method for thefabrication of AAO substrates with pore diameters below 20 nm, and evenbelow 10 nm. In one embodiment, the method entails the use of dilutedelectrolytes at temperature below about 0° C. to form reduced porediameters. According to another embodiment, the present inventionadvantageously provides a method for the fabrication of AAO substrateswith pore diameters greater than 200 nm. The method can include the useof additives that allow anodization voltages in excess of about 200Vduring anodization.

Table 1 illustrates representative ranges of anodization conditions thatcan be used to produce AAO substrates for support structures, and theresulting support parameters in accordance with the present invention.

Both potentiostatic and galvanostatic modes of anodization can beutilized to form the AAO support structure. Potentiostatic mode canensure substantially uniform pore diameter through the thickness of thesupport, with anodization current density and AAO growth rate decreasingwith time. On the other hand, galvanostatic mode maintains a constantgrowth rate, thus ensuring shorter process duration while allowing thevoltage (and the pore diameter) to increase with time.

TABLE 1 Thick- Voltage or Temp. Charge Pore dia., nm/ ness ElectrolyteCurrent (° C.) (C/cm²) Porosity, % (μm) 0.01-5% 5-100 V  −5/+25 2-1000  8-80/10-15 1-500 H₂C₂O₄ 1-50 mA/cm² 0.01-5% 100-600 V  −5/+25 2-1000100-600/10-30 1-500 H₂C₂O₄ + (up to 1000 nm additives** with additionalpore etching) 0.01-3M 5-25 V  −5/+25 2-1000  10-20/7-18 1-500 H₂SO₄0.01-3M 5-25 V −50/+25 2-1000  5-15/5-15 1-500 H₂SO₄* 0.01-3M 10-200 V −5/+25 2-1000 30-200/7-20 1-500 H₃PO₃ *alcohols, ketones, glycols, orother solvents added to lower the freezing point. **additives toincrease anodization voltage include salts of certain metal cations(such as Al(III), Ti(III), V(V), Zr(IV), Nb(V), K(I) and others; anionsof organic and inorganic acids (such as oxalate, citrate, borate, malateand others), as well as metal complexes with other ligands.

The practical limits of the size of the membrane support structure andthe composite membranes is up to 10″ in two of the planar membranedimension. In one embodiment, circular planar membranes with overalldiameter as large as 150 mm were produced.

After anodization, AAO films 102 are still attached to Al 140 as shownin FIG. 3 and FIG. 4, and have a dense aluminum oxide layer 142separating the porous AAO from Al. In order to make a functionalmembrane, this barrier layer 142 has to be removed. This is done eitherprior to the deposition of the active layer as illustrated in FIG. 3, orafter the deposition of the active layer as illustrated in FIG. 4, as isdescribed herein.

If the separation of AAO support membrane is required prior todeposition of the active layer, this process is carried out after theanodization, and involves electrochemical polarization of the AAO on Alsubstrate using acidic electrolytes such as phosphoric acid orhydrochloric acid, or using basic electrolytes such as sodium hydroxideor potassium hydroxide. This process leads to localized dissolution or“breach” of the barrier layer due to the effect of anodic or cathodicbias and results in free-standing membranes with pores that open on bothfaces.

For operation above about 750° C., blank AAO substrates can be annealedat about 900° C. to 1200° C. prior to deposition of the active layer toconvert amorphous alumina into a thermally stable polycrystalline gamma-or alpha-alumina phase.

To facilitate membrane integration, the present invention can includethe fabrication of membranes supported by an aluminum rim as shown inFIG. 5. According to this embodiment, a portion of the AAO supportremains attached to the Al by exposing only the central portion of themembrane 152 to the separation or “barrier layer breaching” step. In oneexample, the barrier layer at the AAO/Al interface in a central portionof the support can be breached without breaking off from the rest of themembrane. Selective back-etch of the Al substrate can then be performedto expose the central portion of the AAO 152, with the rest of the AAOremaining attached to an Al rim.

In another embodiment, a window 150 in the Al foil is etched prior tobreaching of the barrier layer, which thickness is reduced by using anappropriate anodization voltage profile to create asymmetric membranesand to reduce the resistance of the barrier layer in order to facilitatecreation of active layer by electrochemical deposition to be localizedaccording to the present invention.

The procedure for fabrication of an asymmetric support structure for anasymmetric pore membrane involves a three-step anodization methoddescribed in FIG. 6. The steps can include: (1) growth of the “baselayer” at a constant current or voltage to form the desired thicknessand desired first (larger) pore diameter; (2) reduction of theanodization voltage using a smooth profile to form second (smaller) porediameters, where the pore diameter is directly proportional to theanodization voltage; and (3) a current recovery step at constant voltageto condition the barrier layer to ensure that the subsequentelectrodeposition of the active material (e.g., Pd) results in a uniformactive layer. Steps 1 and 2 can be repeated to create multiple layerswith different pore diameters and pore densities. According to thepresent invention, continuous anodization voltage profiles (Step 2above) can be used to increase the rate of formation of the layer(s)with different pore diameter and density and to better control themorphology in such layer(s).

According to one embodiment of the present invention, the shape of theanodization voltage profile for forming reduced pore diameters andthinning the barrier layer (Step 2), and the current recovery profile tomaximize the deposition uniformity (Step 3) is based on a 3rd degreepolynomial used to calculate the voltage-time (E-t) profile:E=at ³ +bt ² +ct+d

The initial anodization voltage (E1) and final anodization voltage (E2)are used as boundary conditions and are selected from available rangesof anodization conditions shown in Table 1. Adjustable parametersincluded the initial (dE/dt)₁ and the final (dE/dt)₂ potential changerate, and the profile duration (t2−t1). The initial potential slope canbe varied from about −50 V/s to about −0.005V/min, and preferably fromabout −1V/s to about 1V/min. The final potential rate change can be fromabout −50 V/s to about 0 V/s, preferably from about −10V/min to about 0.The duration of the voltage reduction step depends on the voltage changerange and desired pore profile, and can vary from about 1 second toseveral hours.

In one example, 5% oxalic acid electrolyte is used for both thesynthesis of the base layer (galvanostatic mode, 5 mA/cm², finalanodization voltage (E1) of 40V, corresponding to a pore diameter ofabout 37 nm), as well as for the formation of the small pore size layer(voltage (E2) reduced to 4V, corresponding to a pore diameter of about 5nm). The first voltage derivative and the process duration are varied,and the second derivative is set to 0.

Representative voltage reduction anodization profiles are illustrated inFIG. 6 (top), along with the schematic representation of the resultingpore structure. A preferred profile for anodization voltage reductionfrom 40 V to 4 V is found to be (dE/dt)1=−0.0070 V/s and t2=1500 s. Theresulting anodization profile for the entire process (growth of the baselayer, forming an active layer and conditioning the barrier layer) isillustrated in FIG. 6 (bottom). Scanning electron microscope (SEM)images of the resulting asymmetric membrane after electrochemicalseparation from Al (performed as described in U.S. Pat. No. 6,705,152 byRoutkevitch et al.) show uniform pore diameter in the base layer and asmooth, homogeneous and defect-free surface of the active layer.Permeability data confirm that the pores are open.

Other profile shapes based on different algorithms can be implemented.It can be described by either a number of known mathematical functions,or a numerical approximation of an exterimental voltage reductionprofile.

In one embodiment, the pore diameter of the membranes can be increasedby conformal dissolution of alumina from the pore walls in appropriateacidic or basic solutions, such as phosphoric acid, sodium hydroxide,potassium hydroxide, or other solutions that can dissolve alumina.Preferred embodiments include the use of 0.5M H₃PO₄ or 0.1M NaOH attemperatures in the range of 0° C. to 50° C. Preferred etching timesdepend upon the desired initial and final pore diameter and thetemperature, and can vary from about 10 seconds to about 5 hours,preferably from about 5 min to about 180 min.

In one embodiment, the pore diameter of the support structure can bereduced by conformal deposition of materials onto the pore walls.Methods for deposition can include sol-gel, solution impregnation,electroless deposition, electrochemical deposition, chemical vapordeposition, atomic layer deposition and others. One preferred embodimentinvolves atomic layer deposition of oxides, such as alumina, silica,zinc oxide, and other materials.

Active Layer Formation

In one embodiment of the present invention, for the fabrication of H₂separation composite membranes, Pd or Pd alloys are used to form theactive layer, with Cu being used as a sacrificial layer. Both Pd and Cucan be deposited inside the pores of an AAO support using eithercommercially available or custom made electrolytes, forming an activelayer having dense nanoplugs disposed within the pores of the support.Both conventional “DC” electrodeposition methods (potentiostatic andgalvanostatic) as well as “pulse” and “AC” techniques can be used.

For the fabrication of symmetric membranes with the active layer withinthe pores and not on the membrane surface, the electrical contact to thepores can be provided by a dense film of conductive material, such as ametal, deposited onto one face of the support. In one embodiment, theconductive film is a Cu film, such as one having a thickness of at leastabout 100 nm and not greater than about 2000 nm. Such a film can bedeposited onto a face of the membrane by DC plasma sputtering, forexample. Galvanostatic deposition (DC mode, current density inside thepores of from about −1 to about −5 mA/cm²) is a preferred method forfabricating symmetric support structures with a Cu contact to formsacrificial and active layers.

For the fabrication of asymmetric membranes, the contact can be providedby the Al substrate. In this case, the deposition is hindered by thepresence of a dense oxide layer at the interface between the support andthe Al foil. This insulating layer prevents the use of conventionalelectrodeposition methods. The preferred methods in this case are pulse,reverse pulse, and AC potential waveform with a DC offset.

The main electrodeposition parameters that define the length and theuniformity of the nanoplugs in the active layer are the electrolytecomposition, the temperature, the deposition mode, the waveform, thepotential, the current density and the time. These parameters areselected based on the type of AAO support structure and desired activelayer morphology. Other factors affecting the uniformity and the rate ofthe deposition of the active layer include the shape and duration of thevoltage reduction profile during the step of the conditioning of thebarrier layer for asymmetric membranes with Al as a contact. A propercombination of these parameters can lead to the electrodeposition offully dense and conformal nanoplugs inside the pores of the AAO supportstructure of both types, symmetric and asymmetric.

One embodiment of the present invention utilizes a 20 Hz to 100 Hz sinepotential waveform having amplitude from ±1V to ±15V, which can dependupon E2. In one embodiment, E2=4V, and the preferred amplitude is ±9V.The DC offset can be from 0 to ±10V, and in one example for E2=4 V, thepreferred DC offset is −1V. The duration of deposition can be from 1second to several hours, with the length of nanoplugs (thickness of theactive layer) generally increasing with the deposition time, as shown inFIG. 7. The preferred duration of deposition of the Cu sacrificial layeris from about 100 s to about 5000 s, forming Cu nanoplugs having alength of from 0.2 μm to 10 μm. A preferred duration of Pd active layerdeposition is from about 200 s to about 4,000 s, resulting in Pdnanoplugs having a length from about 100 nm to about 1 μm, which issignificantly thinner than the Pd film thickness utilized inconventional supported membranes.

Accordingly, electrodeposition can be used to place sacrificial Cunanoplugs at the bottom of the AAO pores, which can be used to localizePd nanoplugs at a desired distance from the membrane surface (FIG. 3,FIG. 4, top chart in FIG. 7).

The sacrificial Cu layer can be removed from the support using knownliquid or gas phase methods for Cu metal etching. For example, anammonium persulfate (APS) etch, or a solution of sulfuric acid andhydrogen peroxide (Piranha etch) can be utilized to remove the Cusacrificial layer. Thus, control of the location of the active layer,such as Pd, within the membrane support can be achieved by usingsacrificial nanoplugs, such as Cu, of varying length.

The present invention includes several methods to form an active layerfrom Pd-alloys. One preferred method is electrochemical co-deposition ofPd and another metal, such as Cu or Ag, from mixed electrolytescontaining ionic species of both Pd and the other metal. The alloycomposition is determined by the partial current densities, which dependon deposition potential and the electrolyte composition.

According to another embodiment of the present invention, a method isprovided that includes annealing of a porous support containing two ormore metals, such as Pd and Cu, that have been consecutively depositedto form an alloy phase between the metals. The alloy composition of theannealed active layer can be controlled by varying the length of themetal nanoplugs, along with varying the annealing temperature and theanneal duration. This approach provides a convenient route forfabricating composite membrane from alloys such as Pd—Cu and Pd—Agalloys, and is also useful for the fabrication of other alloys. Theannealing temperature can be at least about 200° C. and is preferablynot greater than about 1600° C., such as from about 500° C. to about1000° C. The annealing atmosphere can be varied and can include reducingor oxidizing atmospheres, as well as otherwise inert or reactiveatmospheres. Proper selection of the annealing atmosphere can be used toimprove the performance of the active layer due to changes in thechemical composition and the crystalline structure of the active layer.The deposition method for the consecutive metal layers can includesol-gel, solution impregnation, chemical vapor deposition, atomic layerdeposition and other methods.

According to another embodiment of the present invention, consecutivedeposition of different materials and/or different methods as describedabove can be used to form multiple active layers, or to implement layershaving multiple functionality, as shown in FIG. 2. In one preferredembodiment for H₂ separation, the multilayer structure includes apoison-resistant layer, such as Ta for resistance to sulfur, on top of aH₂-separating layer based on Pd or Pd alloys. Another preferredembodiment includes a conformal coating of an appropriate catalyst suchas Cu/ZnO to form catalytic nanotubes adapted to catalyze a steamreformation or water gas shift reaction. Dense nanoplugs of Pd or Pdalloys or other materials described herein, serving a catalytic functionor a H₂ separation function, can form a nanochannel arraymembrane-reactor for converting alcohols or hydrocarbons into H₂. Suchcatalytic coatings as Cu/ZnO can be deposited by sol-gel oratomic/chemical vapor deposition.

In yet another embodiment, the dense barrier layer of AAO can be used asone of the components, or the only component, of the active layer. Inyet another embodiment, the entire anodic alumina membrane or just thebarrier layer can be chemically converted into a different material toform membrane of desired composition.

Fabricating a Membrane Having a Metal Rim

According to one embodiment, the present invention also provides amethod for the fabrication of an AAO support structure having an Al rim.Such a structure is advantageous, as it enables a device fabricated fromthe AAO support, such as a composite membrane, to be sealed into anapparatus incorporating the device.

After forming an optional sacrificial layer 144, and an active layer 110inside the pores of an AAO support structure, three additionalprocessing steps (FIG. 5) can be carried out to fabricate the structurewith an Al rim 112:

1) Etching of Al substrate 140 to open a window 150 in Al;

2) Etching of the barrier layer to provide access to the active layer;and

3) Etching of the sacrificial layer 144, if a sacrificial layer is used.

In one embodiment, the first two steps are performed before forming theactive layer in AAO membrane.

For the Al etching step, the back side of an Al foil with an attachedAAO substrate is masked with a chemically resistive protective pattern,such as by using a photoresist, lacquer or tape, or is patterned with achemically inert gasket or O-ring, to expose an Al area 150 of thedesired size. A wet chemical etch that does not react with the AAO, suchas a solution of hydrochloric acid (HCl) and copper chloride (CuCl₂), isused to dissolve the Al and expose the AAO support structure. Theetching of Al effectively stops when the AAO surface is reached.

To etch the barrier layer 152, if the blank AAO support membrane isformed without breaching of the dense barrier layer, the barrier layerhas to be removed. This process is critical for maximizing membranepermeability, while avoiding overetching of the membrane and maintainingzero defect density. Etching can take place using appropriate acidic orbasic solutions that can dissolve alumina, such as phosphoric acid,sodium hydroxide or potassium hydroxide. In a preferred embodiments, thealumina barrier layer is etched with 0.5M H₃PO₄ or 0.1M NaOH at atemperature in the range of from about 0° C. to about 50° C. Thepreferred etching time depends on the barrier layer thickness andetching temperature, and varies from about 10 s to about 2 hrs. Etchingof the barrier layer can also be performed in a gas phase using, forexample, a plasma ion etch.

Etching of the sacrificial layer 144 can be performed using common wetor gas phase methods known to those skilled in the art, depending on thematerials used as a sacrificial layer. In one preferred embodiment,solution of ammonium persulfate or Piranha etch is used for etching acopper sacrificial layer.

EXAMPLES

Having described the invention, the following examples are given tofurther illustrate the invention. These specific examples are notintended to limit the scope of the invention described in thisapplication.

Example 1 Blank Symmetric and Asymmetric Membranes

Blank AAO symmetric and asymmetric support membranes are formed byanodizing 99.99% pure Al foil that is rolled and pressure-annealed at350° C. and 5,000 psi for 20 min. The resulting Al foil is cleaned andanodized on both sides in 1% oxalic acid electrolyte at a temperature of10° C. and an anodization current density of 10 mA/cm², until a chargedensity of 20 C/cm² is accumulated. The resulting layer of aluminumoxide is then stripped in a hot solution of 200 g/l chromic oxide in 50%phosphoric acid, the Al substrate is rinsed and dried, and an adhesionlayer of 0.5 μm of AAO is grown using the same conditions.

Conventional photoresist is applied to both sides of the Al substrate,is soft-baked at 90° C. for 20 min and is exposed to a UV light using amask with the openings of required size and format to define the number,the location, the size and the format of the membranes—in this case,four 25 mm circular membranes on each side of a 70 mm×70 mm substrate.Final anodization is carried out in 1% oxalic acid electrolyte attemperature of 10° C. and anodization voltage of 80V until chargedensity of 100 C/cm² is accumulated, corresponding to a 50 μm thicksymmetric AAO support structure with a pore diameter of about 65 nm anda porosity of about 12%.

Voltage reduction profile #2 (described above) is applied to some of theAl substrates to fabricate asymmetric support structures with a thinnerbarrier layer and smaller final pore size. The anodization voltage isreduced to 20V and the anodization is continued for another 100 seconds,resulting in a final pore diameter of about 18 nm. Some of these Alsubstrates are rinsed and transferred into a 1M solution of sulfuricacid, where anodization is re-started at 20V, and a different voltagereduction profile is applied to bring the anodization voltage to 2V,resulting in a final pore diameter of less than 5 nm and a porosity ofabout 15%.

To form free-standing membranes without an Al rim, both types of AAOmembranes are separated in a solution of concentrated perchloric acidand acetic anhydride at a cathodic bias of 5 V to 10 V above the finalvalue of anodization voltage. The pore diameter in some of the symmetricmembranes was increased by slow chemical dissolution of the alumina fromthe pore walls for 20 min in the solution of 0.5M phosphoric acid,resulting in the final pore diameter of about 80 nm. The resultingmembranes are rinsed, dried, annealed to 1100° C. to form alpha-alumina,and are then ready for the deposition of active layers. Blank supportmembranes with overall diameter as large as 150 mm were produced in thisexample.

Example 2 Blank Symmetric and Asymmetric Membranes with an Al Rim

Blank AAO symmetric and asymmetric membranes are produced using Al foilprepared and patterned as noted in Example 1, except only one size of Alsubstrate is patterned with 13 mm membranes. Anodization is carried outin 3% oxalic acid electrolyte at a temperature of 12° C. and ananodization voltage of 40V until a charge density of 200 C/cm² isaccumulated, resulting in 100 μm thick AAO films with 37 nm pores. Withsome Al substrates, voltage reduction profile #2 (described above) isused to bring the anodization voltage down to 4 V, and anodization iscontinued for 100 seconds at 4V. The resulting asymmetric AAO has afinal pore diameter of about 5 nm.

The resulting AAO supports, which are still attached to Al, are maskedwith 3M electroplating tape to define 8 mm circles in the center of the13 mm membranes. The barrier layer in the exposed area was breached in asolution of concentrated hydrochloric acid at −2° C. by slow ramping ofthe cathodic potential until 3V to 10V above the final value of theanodization voltage is reached. As a result, the barrier layer isbreached only in the defined 8 mm area, and the rest of the AAO supportremains firmly attached to Al. A backside of the Al foil opposite the 13mm AAO is then also masked to define a 10 mm circles opposite to thearea where the barrier layer is breached, and exposed Al is etched usinga solution of 20% hydrochloric acid and 15% CuCl₂ in water, until theback side of the AAO support was exposed, forming both symmetric andasymmetric membranes with through porosity and supported by an Al rim. Asimilar procedure can be applied to form tubular AAO membranes.

Example 3 Composite AAO/Pd Membranes with Al Rim for H₂ Separation

Blank AAO membranes on Al foil are produced as previously noted inExample 2, except the process is stopped before masking for breaching ofthe barrier layer. Electrodeposition of a Cu sacrificial layer iscarried out in an aqueous solution of 0.5M CuSO₄ for 1000 seconds inpotentiostatic mode using a 100 Hz sinusoidal waveform with an amplitudeof ±9V and a DC offset of −0.5V. Electrodeposition of the active layerof Pd nanoplugs is carried out in a commercial PallaSpeed electrolyte(Technic) for 500 seconds in potentiostatic mode using a 100 Hz SINEwaveform with an amplitude of ±9V and a DC offset of −0.5V. A backsideof the Al foil opposite to the AAO support is masked to define an 8 mmcircle and exposed Al is etched using a solution of 20% hydrochloricacid and 15% CuCl₂ in water, until the backside of AAO/Pd/Cu membrane isexposed. The barrier layer is etched for 20 to 30 minutes in a solutionof 0.5M of phosphoric acid, followed by the selective etch of Cu in theAPS etchant for 2 minutes. This resulted in a 0.6 μm thick active layerof Pd nanoplugs located within the pores approximately 5 μm from themembrane surface. Longer Cu deposition time can lead to the active layerbeing located deeper within the support structure. Increasing the Pddeposition time can result in a thicker active layer. The Al substrateis trimmed to 25 mm diameter.

Testing of both the blank AAO support and the composite AAO/Pd membranesdemonstrate that, when properly supported, the membranes withstandpressures up to 100 psi, and possibly higher, as 100 psi is the upperlimit of the test system. Further, temperatures up to 650° C. (formembranes with an Al rim) and 850° C. (for membranes without Al rim) canbe withstood.

The composite AAO/Pd membranes demonstrate H₂ permeance of up to 0.8mmol/s/m²/Pa^(0.5) at 250° C. to 350° C. (FIG. 8), and permselectivity(i.e., the ratio of permeability of hydrogen to argon) up to 1000.Comparison of blank and composite membrane permeability show that AAOwill not limit the composite membrane performance for the targetedlength of Pd nanoplugs. Composite AAO/Pd membranes also demonstrateexcellent resistance to repeated thermal cycling from 200° C. to 400° C.in the presence of H₂, with no noticeable impact on permeability,permselectivity and membrane integrity (FIG. 9). No negative effects ofH₂ embrittlement and no leaks are detected as a result of temperaturecycling. SEM photomicrographs do not reveal any changes in nanoplugmorphology, which remains conformal to the pore walls.

Example 4 Composite AAO/(ZnO—Cu)/Pd Membrane-Reactor for MethanolReforming

Composite AAO/Pd membranes on Al foils are fabricated as previouslynoted in Example 3. A thin layer of a steam reforming catalyst (ZnOdoped with Cu) is applied to the pore walls by dip-coating in a 0.5Msolution of copper and zinc acetates in isopropanol, followed byblotting of excess solution, drying at 100° C. for 5 min and burn-outfor 10 min at 350° C. Following 5 to 20 catalyst coatings, membranes areannealed up to 750° C. for 1 hr to stabilize the composition and thecrystal structure of the catalyst, and reduced at up to 500° C. for 1 hrin 5% H₂ in Ar to form nanostructured Cu catalyst particles. Thesenanochannel membrane-reactors are tested for H₂ generation by steamreforming of methanol and demonstrated space velocity superior to bulkcatalysts.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention.

What is claimed is:
 1. A planar composite membrane, comprising: a planarporous support structure comprising anodic aluminum oxide and having afirst major surface and a mutually opposed second major surface, theporous support structure comprising substantially parallel elongate porechannels extending through the support structure from the first majorsurface to the second major surface; an active layer comprising anactive layer material, the active layer being disposed within theelongate pore channels between the first major surface and the secondmajor surface; and an aluminum rim integrally formed with an outerperipheral edge of said porous support structure, wherein the aluminumrim is directly bonded to the porous support structure without the useof an intermediate bonding material.
 2. The composite membrane recitedin claim 1, wherein said active layer is completely disposed within theelongate pore channels.
 3. The composite membrane recited in claim 2,wherein said active layer is spaced inwardly from each of said first andsecond major surfaces.
 4. The composite membrane recited in claim 3,wherein said active layer is spaced inwardly from each of said first andsecond major surfaces by at least about 1 nm.
 5. The composite membranerecited in claim 1, wherein said porous support structure has an averagethickness of at least about 0.1 μm and not greater than about 500 μm. 6.The composite membrane recited in claim 1, wherein said active layer hasa thickness of not greater than about 5 μm.
 7. The composite membranerecited in claim 1, wherein said active layer comprises Pd.
 8. Thecomposite membrane recited in claim 2, wherein said active layercomprises Pd alloyed with at least one element selected from Cu and Ag.9. The composite membrane recited in claim 1, wherein said active layercomprises an amorphous metal alloy.
 10. The composite membrane recitedin claim 1, wherein said active layer comprises a first active layermaterial and a second active layer material that is different than saidfirst active layer material.
 11. The composite membrane recited in claim10, wherein said first active layer material is adapted to separate agas species.
 12. The composite membrane recited in claim 11, whereinsaid second active layer material is a catalytic material.
 13. Thecomposite membrane recited in claim 1, wherein the porous supportstructure and the aluminum rim are integrally formed by the anodizationof a single sheet of aluminum foil.